This invention relates to an imaging apparatus having a function of correcting a dark output of an imaging device.
Recently, in order to image and record mainly still pictures, electronic still cameras are developed. In video cameras for recording moving pictures, a still picture imaging/recording function is added. When photographing still pictures by use of the above cameras, the so-called long-time exposure technique in which the exposure time is elongated by making long the charge storage time of the imaging device is used. In the long-time exposure technique, the photographing can be attained without using an auxiliary light source such as a strobe light in a low illumination environment.
In the imaging device, since a dark output due to a so-called dark current exists, there occurs a problem that the dark output is superposed on an image signal to deteriorate the image quality. As a method for compensating for deterioration of the image quality due to the dark output, a method for using output information of the pixel and subtracting the dark output level from the output level of the pixel signal to derive only the signal component is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 60-53383, for example, and is known in the art. In this specification, a process for eliminating the dark output component from the deteriorated signal output of the pixel is referred to as a dark output correction process. The correction process is excellent in principle in that original pixel information can be derived without causing deterioration in the resolution.
However, if the dark output correction process is simply effected by use of the conventional technique, a false signal is created by the correction process and deterioration of the image quality occurs when the dark output level is high since the output range of the imaging device used in the present electronic camera is limited. This is explained by taking a concrete case as an example. It is assumed that a dark output of high level (which is set at 50% of the saturation level of the imaging device) is generated from a pixel concerned. In this case, it is possible to eliminate the dark output by subtracting the dark output component by effecting the dark output correction process, but the output signal of the pixel concerned is 50% of an original subject luminance level at maximum. That is, since the saturation level of the pixel concerned is lowered, a so-called black scar occurs when the subject to be actually photographed is bright.
More specifically, since the dark output correction process has the above problem, deterioration of the image quality due to the dark output is conventionally compensated for mainly by the pixel defect compensating technique. The pixel defect compensating technique is to determine a deteriorated pixel whose dark output level is higher than a preset level as a “defective pixel”, make information of the pixel invalid and use nearby pixel information instead of the deteriorated pixel information. The pixel defect compensating technique is extremely effective when only a small number of “defective pixels” independently exist.
However, in this type of pixel defect compensating technique, since the pixel information of the deteriorated pixel is made invalid and discarded, the resolution is locally lowered in principle. When dark output levels of a large number of pixels are increased over the entire portion of the image plane, particularly, at the high temperature or at the time of long-time exposure, the number of substantially effective pixels is extremely reduced, and as a result, the image quality will be extremely deteriorated.